add medium field for eBooks to MLA 8th

POBrien333 · web-flow · commit 409017c35b2b · 2021-04-08T12:37:02.000+02:00
via https://forums.zotero.org/discussion/70629/entry-type-for-ebook Not sure if we want this, but this is a suggestion.
diff --git a/modern-language-association.csl b/modern-language-association.csl
@@ -138,6 +138,7 @@
         </else>
       </choose>
       <text variable="version"/>
+      <text variable="medium"/>
     </group>
   </macro>
   <macro name="volume-lowercase">
